A magnetostriction type torque sensor is widely known as an instrument for detecting torque applied to a rotary shaft. The magnetostriction type torque sensor is configured to have two upper and lower annular magnetostrictive films formed on the outer peripheral surface of the rotary shaft, and is configured to detect torque of the rotary shaft by detecting a change in the magnetostrictive characteristic of the upper and lower magnetostrictive films which change is caused when the rotary shaft is torsionally deformed. In order to secure the detection accuracy of torque, it is necessary that the axial-direction thickness of the magnetostrictive film is uniform, and that the alloy composition of the magnetostrictive film is uniform.
Conventionally, there is known a method shown in FIG. 5 as a method of applying plating to the surface of a shaft-shaped member used as a rotary shaft (Japanese Patent Laid-Open No. 2005-3622).
In a plating apparatus 101 disclosed in Patent Document 1, a shaft-shaped member 104 serving as a cathode and a Ni—Fe plate 105 serving as an anode are immersed in a plating liquid 103 stored in a plating tank 102.
Three cylindrical shielding tools 106a, 106b and 106c are mounted on the outer peripheral surface of the shaft-shaped member 104. Further, a masking tape 107 is wound around portions which are respectively located on the upper side of the upper end shielding tool 106a and on the lower side of the lower end shielding tool 106c, and in which the outer peripheral surface of the shaft-shaped member 104 is exposed.
Here, when a current is made to flow between the shaft-shaped member 104 and the Ni—Fe plate 105, Ni ions and Fe ions are dissolved into the plating liquid 103 from the Ni—Fe plate 105. By the Ni ions and Fe ions in the plating liquid 103, Ni—Fe-alloy plating 108 is applied to the portions of the shaft-shaped member 104 (hatched portions in the figure), which portions are not covered with the shielding tool 106 and the masking tape 107. Thereby, two magnetostrictive films 108 are formed on the upper and lower sides on the outer peripheral surface of the shaft-shaped member 104.
However, in the plating apparatus 101 as described in Patent Document 1, there is no liquid flow in the plating liquid 103 in the plating tank 102. Thus, the capacity for supplying the Ni ions and Fe ions dissolved in the plating liquid 103 to the shaft-shaped member 104 is reduced, and thereby a high current density cannot be maintained. This results in a disadvantage that the plating rate cannot be increased. Further, the Ni ion concentration and the Fe ion concentration in the plating liquid 103 are lowered on the surface not facing the Ni—Fe plate 105 of the shaft-shaped member 104. This results in a disadvantage that there occur abnormal deposition in which the composition, and the like, of the magnetostrictive film 108 is made non-uniform.